1. Field of the Invention
The present invention generally relates to equalizing amplifiers, and more particularly to an equalizing amplifier and a receiver using the same. Further, the present invention is concerned with a preamplifier suitable for such a receiver.
In optical transmission systems conforming to the CCITT recommendations, there are two types of transmissions, namely, a short-distance transmission connecting switching frames together, and a long-distance transmission connecting switching stations together. It is desired to provide an equalizing amplifier capable of shaping a waveform deformed in a transmission path of either the short-distance transmission or the long-distance transmission.
2. Description of the Prior Art
FIG. 1 is a block diagram of a structure of a conventional light receiver for optical transmission. The receiver shown in FIG. 1 is primarily made up of an equalizing amplifier and a decision circuit.
The equalizing amplifier is connected to a light receiving element 11a and a preamplifier 12, and is comprised of a limiter 13, an averaging circuit 14, and an amplifier 15. The light receiving element 11a is formed of, for example, a PIN photodiode which converts a light signal transmitted over the optical transmission line into an electric signal. The preamplifier 12 has a feedback resistor Rf with a diode connected in parallel therewith, and amplifies a fine current signal S1 from the light receiving element 11a. The limiter 13 has the function of amplifying an output signal S2 of the preamplifier 12 to a level which can be decided at a following stage. The averaging circuit 14 receives a non-inverted signal S3 and an inverted signal S4 from the limiter 13, and detects the respective average values. The amplifier 15 amplifies the output signals of the averaging circuit 14, and generates therefrom a threshold voltage S5 to be applied to the limiter 13. The averaging circuit 14 and the amplifier 15 form a DC offset compensation circuit.
The decision circuit is made up of a band-pass filter (BPF) 16, a limiter 17, and a decision (discrimination) circuit 18. The band-pass filter 16 extracts a second-harmonics component (2fo) of the inverted signal S4 which is one of the output signals of the limiter 13. The limiter 17 receives an analog signal from the filter 16, and generates therefrom a non-inverted clock signal and an inverted clock signal. The decision circuit 18 receives the non-inverted signal S3 and the inverted signal S4 from the limiter 13, and decides data using the non-inverted clock signal and the inverted clock signal from the limiter 17.
A description will now be given, with reference to FIG. 2 which is a waveform diagram, of the operation of the light receiver shown in FIG. 1. The light receiving element 11a converts the light signal into the current signal S1 (part (A) of FIG. 2) and applies it to the preamplifier 12. The preamplifier 12 amplifies the received current signal S1 by i.times.Rf where i is the value of the current signal S1, as shown in part (B) of FIG. 2.
The output signal S2 of the preamplifier 12 is applied to the non-inverting input terminal of the limiter 13, which outputs the non-inverted signal S3 (solid line) having an amplitude of i.times.Rf.times.G where G is a predetermined gain (equal to a maximum gain which can be decided by the decision circuit 18) and outputs its inverted version S4 (dotted line), as shown in part (C) of FIG. 2.
The averaging circuit 14 receives the non-inverted signal S3 and the inverted signal S4, detects the respective average values S3' and S4', as shown in part (d) of FIG. 2, and generates the threshold voltage S5 equal to the difference between the average value S3' and the average value S4'. The limiter 13 is controlled by the above threshold voltage S5.
As described above, the current-voltage converting rate as well as the receiving level (receiving sensitivity) of the preamplifier 12 mainly depend on the value of the feedback resistor Rf. Hence, it is necessary to determine the value of the feedback resistor Rf so that a desired receiving level can be obtained.
FIG. 3 is a circuit diagram of a circuit configuration of the preamplifier 12 equipped with the feedback resistor Rf as described above. When it is assumed that the diode D10 connected to the feedback resistor Rf is omitted from the circuit, the preamplifier operates as follows. At the time of receiving no light input signal, a base current flows in a transistor Tr2 via a resistor Rc connected to the base thereof, a diode D12 and an emitter resistor Ro coupled to the emitter thereof. The base current also flows in a diode D11 from the base of the transistor Tr1 via the feedback resistor Rf. That is, a feedback loop system is formed so that the currents always flow in both of the transistors Tr1 and Tr2. This feedback loop system functions to make the circuit operation stable and outputs a predetermined voltage to the output terminal of the preamplifier.
When a light signal is received in the above state, the current S1 flows from the light receiving element 11, and a current flows in the feedback resistor Rf and the emitter resistor Ro. Hence, the voltage of the output terminal is a divided voltage due to the feedback resistor Rf and the emitter resistor Ro. Normally, the feedback resistor Rf is designed to have a large value in order to suppress noise, and hence a small voltage is obtained at the output terminal. Thus, if the light input signal has a high level, the transistor Tr2 may be saturated (cut off), and the normal feedback operation cannot be obtained.
In order to overcome the above disadvantage, as shown in FIG. 3, the diode D10 is connected to the feedback resistor Rf in parallel in order to perform voltage clumping due to a voltage drop of 0.8 V developing across the diode D10. The voltage drop by means of the transistor Tr1 and the diode D11 is equal to 1.6 V (equal to 0.8 V+0.8 V), and the voltage across the emitter resistor Ro is always maintained at 0.8 V equal to 1.6 V-0.8 V, as shown in FIG. 3. In the above way, the transistor Tr2 can be prevented from being cut off.
The diode D10, which is connected to the feedback resistor Rf in parallel in the preamplifier 12 connected to the equalizing amplifier of the light receiver, causes a problem when the light input signal (dotted line) having a large receiving level as shown in FIG. 4 is received. This problem is such that the output level of the amplifier is clamped at 0.8 V, as shown in FIG. 4 and hence the pulse width thereof greatly deviates from a mark/space ratio of 1/2. As described before, the limiter 13 functions to amplify the signal S2 to the maximum level that can be decided by the decision circuit 18 of the following stage. When the threshold voltage S5 applied to the limiter 13 is controlled by the error (difference) shown in part (D) of FIG. 2, the threshold voltage S5 behaves, as shown in part (B) of FIG. 5, in a way different from that obtained when the light input signal at a minimum level is received (part (A) of FIG. 5). That is, the threshold voltage S5 shifts toward the lower level ("0") as shown in part (B) of FIG. 5. Hence, the light receiver is liable to be affected by noise which may be caused due to various factors as well as variations of the received signal.